How lowering the thermostat lowers your heating bill

House Doctor | Gary Dymski

January 27, 2005

It is common knowledge that energy use can be reduced by turning down the thermostat at night. However, I am not sure how this saving is accomplished. I understand that more energy is used to heat a house to 70 degrees than to 65. However, if the thermostat is set to 70 degrees, and the temperature drops to 69, the furnace clicks on and raises the temperature back to 70. There is a measure of energy used to raise the temperature one degree, from 69 to 70. But isn't the same energy used if the thermostat is set at 65 and the furnace clicks on when the temperature drops to 64? So here's my question: If the energy needed to keep a constant temperature is simply the amount required to raise the temperature 1 degree, why is it more economical to keep it at 65 instead of 70? Or even 75? The only difference, it seems, is the initial energy used to raise the temperature. Mike C., Stony Brook

According to the Energy Ideas Clearinghouse (www.energyideas.org), every degree a temperature setting is increased in winter adds about 3 percent to the average home heating bill. Every degree cooler a temperature setting is decreased in summer adds about 4 percent to the average home air-conditioning bill.

The same group says that lowering a thermostat setting from 70 degrees to 65 degrees for eight hours a day - the time a house is unoccupied - can reduce a heating bill by about 5 percent. To get the same reduction in the summer, turn the air-conditioning up five degrees, say from 70 to 75.

The idea that leaving the thermostat at a steady temperature requires less energy, which is what you imply, is a myth. Research has proved that the energy required to reheat the house is nearly equal to the energy saved as the temperature cools. Savings are realized in the time it takes for the indoor temperature to reach the lower thermostat setting to when heat clicks on again.